1. Field of the Invention
The present invention relates to electrical transducers and, in particular, to an actuator that forcefully positions in a translatory direction perpendicular to the broad plane of the actuator.
2. Description of the Related Art
A diverse class of known actuators, comprising integral piezoelectric body portions that forcefully position an object in two or more directions, use the thickness or extension piezoelectric mode in at least one said portion. When thickness or extension actuator portions are affixed to a rigid structural member, a significant fraction of the mechanical stroke and available force is relegated to the generation of internal strain because thickness and extension deformations are inseparable. The extending (actually contracting) portion is under tension, a strain less well accommodated by brittle ceramic materials. Internal strain hastens dissolution of a highly stressed actuator. Thickness and extension deformations, being inseparable, cause one motion to influence the other. No known form of ferroelectric actuator using an electric field parallel to the polarization direction is capable of producing a pure translation throughout the actuator body.
A thickness or extension mode piezoelectric element is generally polarized by applying an electric field across its layer thickness (shortest distance). Later, the application of an operating electric field in the same direction as the original polarizing field results in the thickness or extension deformation. Large fields are required to generate desirably large deformations. Thickness and extension actuators are generally restricted to monopolar electric drive. If an intense electric field is applied in a direction antiparallel to the direction of original polarization, the polarization is reduced, destroyed, and in severe cases, reversed to varying degrees. The latter is unacceptable in all applications wherein the sense of action must be preserved relative to the polarity of the applied electric potential. It is highly desirable for the piezoelectric actuator to accommodate bipolar electric drive. Electric and electromechanical resonance is characterized by symmetric sinusoidal voltage swings. Common thickness and extension piezoelectric elements accept bipolar drive when the potential-supplying apparatus is elevated toward one polarity with a potential high enough to avoid depolarization. The value of potential commonly used is half the maximum peak-to-peak swing. Elevated potential, also called floating, power supplies require more robust insulation, and leave some circuit portions at high potential, a hazard. Circuits using the capacitance of the piezoelectric element as a portion of a floating free-running resonator are therefore more difficult to design and construct. Rectification of an otherwise polarity-symmetric drive leaves the actuator undriven during each "wrong" half cycle.
Monopolar electric drive causes a predetermined piezomechanical stroke associated with a particular applied electric field. When the actuator accepts the opposite polarity of electric field without depolarizing, the actuator may provide another increment of stroke of equal magnitude but opposite direction. Therefore, bipolar drive effectively doubles the available mechanical stroke without raising either the applied electric field intensity or the state of maximum strain.
An advantage of the thickness mode actuator is the ability to apply relatively large forces in the stroke direction. This direction is normal to the broad surfaces of the sheets of piezoelectric materials generally used. Normal force places the piezoelectric layers under compression, a force particularly easily accommodated by brittle piezoelectric ceramics.
A diverse class of actuators, particularly those executing smooth traction walking, benefit from non-sinusoidal mechanical strokes. Non-sinusoidal strokes preclude the direct application of sinusoidal resonant electric drive. Slowly varying direct current electrical sources, such as programmable DC power supplies, for example, have been used to control piezoelectric positioners. These power sources emulate class A amplifiers but have a restricted frequency response. Piezoelectric actuators are almost completely capacitively reactive. All the reactive capacitive current flows through the amplifier output devices. A class A amplifier dissipates all of the available power internally under null excitation, a detriment to electromechanical efficiency. The variable DC voltage is essentially free of superimposed high frequency ripple, and it provides smooth control and piezoelectric positioning at slow speed (including zero speed) with relatively good positioning accuracy. At modest frequencies and voltages, programmable DC power supplies operate piezoelectric actuators as smooth walking actuators without losses from sliding friction. However, high efficiency is beyond the capability of a programmable DC power supply emulating a class A or class B amplifier. Operation becomes more difficult and less efficient above a few walking cycles per second or with voltages above about 200 volts. Furthermore, none of the known class A or class B linear amplifiers remain stable when driving an entirely capacitive load. Therefore, they are not applicable to electromechanically efficient piezoelectric walking actuators except at the lowest portion of the amplifier's frequency band.
A electronic drive described in U.S. Pat. No. 4,628,275 issued to Skipper, et al. emulates a class A amplifier. The amplifier provides the high bipolar voltage swings necessary to operate piezoelectric shear actuators. However, the ultrasonic charge transfer cycles of the amplifier, even when holding a steady voltage, accelerate the rate of wear and fatigue in all mechanisms connecting the actuator to positioned objects. Furthermore, the amplifier requires high voltage DC power supplies, large and heavy transformers, and very fast switching devices to achieve modest electrical efficiencies. The use of AC-to-DC power converters and the presence of large reactive currents in output switching devices precludes efficiencies above about 60%.
Piezoelectric actuators capable of smooth walking are inherently well suited to operation in a vacuum, such as in orbiting space stations, because lubrication is not required. The high mechanical efficiency of piezoelectric actuators also avoids excessive heating during operation, thus eliminating the need for ancillary cooling that reduces overall system efficiency. Furthermore, piezoelectric actuators require no additional energy from the power source to maintain a stationary force.
Operation in a vacuum imposes similar thermal management requirements on the drive electronics. The need for heat removal decreases dramatically with increasing efficiency. Internal heat generation by an ideal electrical power source is negligible when piezoelectric actuators apply a stationary force or operate at low velocities. Ideally, the energy supplied by the drive system should equal the energy converted to useful mechanical work by the actuators. The walking actuator provides a normal force necessary for frictional traction or the engagement of teeth before application of a tangential force by another actuator body portion. Because resonance and bipolar drive offer relatively high efficiency, there is a need for a lifting body portion with these qualities.